Study On Propagation Characteristics Of Laser Excited Surface Acoustic Waves In Periodic Structures

Surface acoustic wave(SAW)is a special type of elastic wave that propagates along surfaces.Since the energy of SAW is mainly concentrated in the wavelength range from the surface,SAW is sensitive to the properties and structures of object's surface.Laser-excited SAW is often used to detect the surface properties or surface defects.At the same time,similar to the case of bulk acoustic waves,there exist SAW band gap in the periodic structures,and the propagation of SAW is forbidden in the band gap frequency range.By constructing defects in the periodic structure,a defect mode can be formed such that the surface acoustic wave can be guided along the defects or localized in the defects.The structural parameters of the unit structures and the defects can be designed according to the actual needs,to guide and modulate SAW and achieve local enhancement in a specific frequency band.In this paper,based on the energy band theory of periodic structure and finite element calculation,a T-shaped unit structure is analyzed in detail,and the energy band characteristics and propagation characteristics of SAW in the surface of the periodic array structure material are studied.The effects of structural parameters and material parameters of the periodic structure on the band structure and band gap position are analyzed.The transmission spectrum of the periodic structure is calculated and the reliability of the energy band calculation is verified.The numerical results show that the SAW will strongly couple with the stacked structures and form local resonant bandgaps.Correspondences between the band position/width and the structural/material parameters of the unit cell are clarified.The propagation characteristics of SAW in the periodic structure with defects are further studied.The calculation results based on the supercell method prove that the defect mode will appear in the defects.As the number of unit structures increases,due to the coupling between the adjacent structures the number of SAW modes increases correspondingly.More importantly,the defect modes appear and acoustic waves will propagate along the defect or be confined to the defect,providing an effective way to manipulate and guide SAW.An full-fiber Fizeau interferometer system is established experimentally,and its detection reliability is verified by experiments.The surface acoustic wave is excited with a 7 ns pulsing laser,and the surface displacement is detected with the fiber Fizeau interferometer.After the signal is collected and analyzed,the group velocity of SAW along a photosensitive resin surface is measured to verify the reliability of the experimental setups,builting a foundation for further experimental researches on SAWs in periodic structures with defects.In this paper,the study on the propagation characteristics of SAW in the periodic structure with defects will provide a reference for the design of novel SAW devices and the guidance and manipulation of laser SAWs.